Preheat oven to 350°F (180°C). Grease, line with parchment, and flour two round 8-inch pans.
In a medium bowl or measuring cup, combine and stir the egg whites, 1/4 cup of milk, and the vanilla. Set aside.
In the bowl of an electric mixer fitted with the paddle attachment, combine the dry ingredients together on low speed for 30 seconds. Add the butter and remaining 1/2 cup of milk, and mix on low speed until just moistened. Increase to medium speed and mix for 1 1/2 minutes.
Scrape the sides of the bowl and begin to add the egg mixture in 3 separate batches; beat on medium speed for 20 seconds after each addition. Divide the batter in two, spreading it evenly with a small offset palette knife. If you have a kitchen scale, weigh to ensure 2 even layers. Bake 25-35 minutes be careful to not overbake. Check cake at 20 minutes, but not before, and once you feel it’s almost ready, set the timer for 2 minute intervals. Let cool on racks for 10 minutes before loosening the sides with a small metal spatula, and invert onto greased wire racks. Gently turn cakes back up, so the tops are up and cool completely.
Whipped Vanilla Bean Frosting
In a mixer whip butter for 8 minutes on medium speed. Butter will become very pale & creamy.
Add remaining ingredients and mix on low speed for 1 minute, then on medium speed for 6 minutes. Frosting will be very light, creamy, and fluffy.
You can swap the vanilla bean and use a total of 1 1/2 teaspoons of pure vanilla extract.